The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typical to subject it to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the subsequently applied radiation-sensitive coating and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining affects both the performance and the durability of the printing plate, and the quality of the graining is a critical factor determining the overall quality of the printing plate. A fine, uniform grain that is free of pits is essential to provide the highest quality performance.
Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Optimum results are usually achieved through the use of electrolytic graining, which is also referred to in the art as electrochemical graining or electrochemical roughening, and there have been a great many different processes of electrolytic graining proposed for use in lithographic printing plate manufacturing. Processes of electrolytic graining are described, for example, in U.S. Pat. Nos. 3,755,116, 3,887,447, 3,935,080, 4,087,341, 4,201,836, 4,272,342, 4,294,672, 4,301,229, 4,396,468, 4,427,500, 4,468,295, 4,476,006, 4,482,434, 4,545,875, 4,548,683, 4,564,429, 4,581,996, 4,618,405, 4,735,696, 4,897,168 and 4,919,774.
In an electrolytic graining process, the aluminum is treated, so as to increase its surface area and create a specific surface structure, by passing an electric current--usually an alternating electric current--from an electrode through an acid electrolyte to the aluminum. Typically, the aluminum that is conveyed through the electrolyte solution is in the form of a thin continuous web that may have a width of as much as two or more meters. It is desirable to grain the surface with a high efficiency in regard to both electric power and chemical consumption, while at the same time achieving proper grain morphology without excessive formation of adhering reaction by-products, commonly referred to as "smut". The presence of smut can necessitate an aggressive etch treatment, following the graining operation, which can further modify the surface in an unwanted manner. It is therefore highly desirable to operate the process in such a way that a minimal amount of smut is formed, and that which is formed is loosely bound and easily removed.
In carrying out electrolytic graining of aluminum, it is typical to utilize nitric or hydrochloric acid in admixture with the respective aluminum salt thereof. Other acids and many other types of chemical agents are also known for use in electrolytic graining baths. Electrodes, most commonly formed of graphite, are positioned to oppose the aluminum web at a distance ranging from about one-half centimeter to several centimeters. Either single phase or three phase alternating current is passed through the electrolyte so that at the interface between the solution and the aluminum, a displacement reaction occurs whereby aluminum is oxidized to form either the chloride or nitrate salt which is soluble in the solution. By removing aluminum with the use of an electric current, a specific surface structure is obtained. Parameters such as temperature, electrolyte concentration, flow rates and electrode spacing are important in determining the characteristics of the surface structure produced.
Most of the known electrolytic graining processes involve the use of uniform current density along the web. However, Oda et al in U.S. Pat. No. 4,272,342 propose a method of electrolytic graining of aluminum in which an alternating current is passed through the aluminum in such a way that EQU Q.sub.1 &gt;Q.sub.2 &lt;Q.sub.3
wherein Q.sub.1, Q.sub.2, and Q.sub.3 represent, respectively, the quantity of electricity per unit area of application during the first one-third period, the intermediate one-third period and the final one-third period of the total electrolytic graining time. This method of control of current density distribution is said to reduce the total quantity of electricity required and to provide improvement in the quality of grain structure realized. However, there is still a critical need in the art for an improved electrolytic graining process which will provide a grain structure that is more ideally suited to the requirements of lithographic printing plates.
It is toward the objective of providing new and improved lithographic printing plates, having an aluminum support with a more uniform electrolytically grained surface, that the present invention is directed.